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Not Only IP Packets in 5G

On high level, the design criteria for 4G (aka LTE or EPS) were simple and straightforward: a system for transporting IP packets between a terminal (the UE) and the edge of the operator network (the PGW), thus connecting the UE with some external Packet Data Network, e.g. the Internet. Yes, and of course we need support for SMS which is natively non-IP. But it was, and still is, basically about doing everything over IP.

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5G packets are like different Christmas presents under the tree!

In 5G, a PDU Session can be set up between the terminal (still called UE) and the edge of the operator’s network where we now find a User Plane Function (UPF). PDU is short for Packet Data Unit, and a PDU can be an IP packet. So far it sounds very much like 4G, right?

But if we study the 5G specifications from 3GPP, we find that there are actually three types of PDU Sessions; IP, Ethernet and Unstructured.

If the PDU Session is of type IP, this means that the 5G System is used for transporting IPv4 or IPv6 packets from UE to (the last) UPF and vice versa. This makes the UPF a (kind of) router since it operates on IP packets for giving the UE Internet access, IMS access or access to some other IP-type Data Network. Pretty much like data traffic in GPRS, UMTS and 4G.

However, if the PDU Session is of type Ethernet, the role of the 5G System is to give the UE layer 2 connectivity (Ethernet) rather than layer 3 connectivity (IP). So, Ethernet frames are sent over the NR radio interface processed by a pile of radio protocols and then transported into the 5G Core using GTP-U tunnels. If this feels a bit quirky, it is probably because we are so used to 2G, 3G and 4G transporting IP packets. But from a more philosophical point-of-view, the 5G System just transports something (a Packet Data Unit) and it is no major difference if this “something” is an IP packet or an Ethernet frame.

What would be the use case for Ethernet type PDU Sessions? Well, maybe it is a little too early to say, but the point in this case is that the 5G UE will be part of a LAN (Ethernet) where the rest of the LAN is placed in e.g. a company or organization somewhere in the Data Network outside the 5G System. Technically, the UPF will act as a (kind of) L2 switch, operating on Ethernet frames. The UE may still have an IP address, and in fact it is very likely to have one. But the IP address would in this case not be allocated by the 5G System (the operator) but e.g. by the company DHCP server.

Perhaps even more quirky is a PDU Session of type Unstructured. In this case the 5G System is used for transporting some Packet Data Units whose format, or structure, is unknown to the 5G System. Since the 5G System does not know e.g. where the header ends and the payload starts – or even if there is header and payload – the 5G network will be totally transparent and just provide a “transport pipe” between the UE and the last UPF. However, the UPF needs to have some information about what to do with PDUs belonging to PDU Session x, y or z. So, some type of mapping between PDU Session ID and external transport address or interface is required.

Use case for this? Well, it is basically a pure bearer service or a serial connection. Lawful intercept will be tricky, if required. Reactions from my students so far range from “interesting” and “looks odd” to “sounds dangerous”. But perhaps we can see this a way to be future proof and open up for new protocols to be used in 5G? Maybe it turns out that IP is not the ideal protocol for all future 5G services? Sooner or later, someone will argue that sticking to an old protocol from the early 1980’s does not make sense anymore. And if not in 5G, perhaps in 6G. For the time being, this type of connection is best suited for tiny IoT devices (e.g. a water meter sending its reading, serial number and battery state, or a garbage bin asking to be emptied) as they do not really need IP connection to browse the Internet or check emails.

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Until next time,

The Apis IP-Solutions Team

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